1. Field of the Invention
This invention relates to a device for splitting a light beam by means of a diffraction lattice (or grating) and for detecting a portion of the light. More particularly, the invention is concerned with such a device adaptable to a light metering device in a photographic camera.
2. Description of the Prior Art
The light metering device in a photographic camera splits a portion of an image-forming light beam which has passed through an object lens by a beam splitter, and the thus split light is detected by a light detector. For a light beam splitter useful for such a light metering device, there has so far been used a single half mirror which is obliquely provided in the camera. Such a beam splitting device for use in the camera, etc. should desirably be as thin as possible. U.S. Pat. No. 3,464,337 discloses a device, in which two pieces of members, each being provided on its surface with a plurality of slanted reflective surfaces, are cemented together to form a prismatic line raster, and a part of the incident light is split in a geometrical-optics manner for light metering.
The present invention is to provide such detecting device having a beam splitter of much thinner thickness by use of a diffraction lattice (or grating) having a small pitch of 10 .mu.m or so.
In the beam splitter to split light using such a diffraction lattice, it is desired that no useless diffracted light should occur, except for an image forming light beam emitted from the diffraction lattice and diffracted light of a particular order for light metering, because such useless diffracted light is harmful to image formation of an object due to flare light or ghost image caused or created thereby.
A beam splitter which has solved such problems is disclosed in the laid-open Japanese patent application No. 53-42042 filed by the assignee-to-be of the present application. The light beam splitter disclosed in this laid-open application uses a relief type diffraction lattice. (A diffraction lattice, on one surface of which a plurality of peak-and-valley structures are regularly formed, is usually called a "relief type diffraction lattice".) The beam splitter as disclosed in the above-mentioned application particularly uses a blazed diffraction lattice, one of the various types of relief diffraction lattices, having such a characteristic that it diffracts almost all quantity of the incident light in one specific diffraction order, whereby more than 90% of the diffraction efficiency of a zero order transmissively diffracted light for the image formation of an object, a few percent of the diffraction efficiency of the primary order diffracted light is available for light metering, and substantially all of the diffraction efficiency of other orders are rendered zero. The reason for using the primary order diffracted light as the light metering diffracted light is that, as described in the laid-open specification, it is difficult for the type of blazed diffraction lattice known at present to increase the diffraction efficiency of two diffracted lights, the diffraction order numbers of which are not adjacent each other, in comparison with the other diffracted lights.
Generally, when a light is to be split in the direction of a diffraction angle .theta. using a diffraction lattice having a pitch d, the following relationship is established between the pitch d and the diffraction angle .theta. of N-order. EQU d sin .theta.=N.lambda. (1)
(.lambda. is a wavelength of incident light)
As will be readily understandable from the above equation, the lower the order number N of the diffracted light for use in the light metering becomes, the smaller become the diffraction angle .theta. and the pitch d. However, the diffraction angle .theta. should be at a certain definite value or above, e.g., 30.degree. or so. On account of this, when a diffracted light for light metering having the diffraction angle .theta. of 30.degree. and the wavelength .lambda. of incident light of 0.55 .mu.m is to be split by means of the beam splitter as shown in the embodiment of the abovementioned laid-open patent application No. 53-42042, for example, wherein the zero order transmissively diffracted light and the primary order diffracted light are used as the image-forming light and the light metering diffracted light (N=1), the pitch d of the diffraction lattice is calculated from the above equation (1) as 1.1 .mu.m, which means that a very fine pitch should be provided on the diffraction lattice.
A diffraction lattice having such fine pitch requires precision manufacturing in the production of a "master" diffraction lattice to be the base and "copy" diffraction lattices causing undesirable increases in the cost of mass-production.
In order therefore to relax the conditions imposed on such a diffraction lattice and to obtain a beam splitter which is easy to manufacture, it has been desired that a diffraction lattice capable of using a high order diffracted light for the light metering be provided.